Chemical mechanical polishing is one of the many steps commonly used in the manufacture of integrated circuits. As detailed in many prior art patents, chemical mechanical polishing, or simply "CMP," is the process of polishing the surface of a semiconductor wafer in order to remove material from the surface of the wafer. The polishing is typically performed by rotating a polishing pad against the semiconductor wafer. A slurry of some sort is used to facilitate the polishing process. Depending upon the material that is to be removed from the semiconductor wafer, the composition of the polishing pad and the composition of the slurry varies.
For example, in the CMP of tungsten material from the surface of a wafer, the slurry will include an oxidizer, which is typically ferric nitrate crystals (Fe(NO.sub.3).sub.3). The ferric nitrate crystals are usually diluted in deionized water and then mixed with aluminum oxide (Al.sub.2 O.sub.3) before being introduced into the CMP apparatus. Examples of slurry compositions are detailed in U.S. Pat. No. 5,783,489 to Kaufman et al. and the patents cited therein.
These slurries shown in the '489 patent and other slurries typically include silica, alumina, ceria, titania, and/or zirconia abrasive particles. These particles are suspended in a liquid naturally or by adding a surfactant. Nevertheless, one known problem with CMP slurries is that, for a variety of reasons, the particulates in the slurry may gel or flocculate. The flocculation may result from a change in pH level, heat, light, sedimentation in the delivery system at low flow rates, shear forces, metal contaminants, and other particle interaction. If this occurs, the agglomerate particles may scratch the surface of the wafer. These defects can result in short circuiting of metal interconnect layers. The defects may be singular or may be of the "skipping stone" type.
Therefore, what is needed is a method of minimizing the amount of agglomeration of particles in the CMP slurry.